Conventionally, the sewing machine driving apparatus uses a driving motor shown in FIG. 1. The driving motor has an induction motor, a clutch and a brake mechanism integrated in construction. A driving shaft 101 of the induction motor, and a fly-wheel 102 connected with it, and a clutch friction plate 103 continuously rotate and store rotational energies in the fly-wheel 102. A clutch moving disc 104 and a brake moving disc 105 may axially slide on a spline 107 pressed into the output shaft 106 and have linings 108 and 109, respectively. Now, suppose a clutch magnet 110 is excited to produce magnetic flux 111, the clutch moving disc 104 moves leftwardly, the lining 108 is brought into pressure contact against a continuously rotating clutch friction disc 103 and the rotating force of the fly-wheel 102 is transmitted to an output shaft 106 through the spline 107. Then, when a brake electromagnet 112 is excited, a brake moving disc 105 moves rightwardly and the lining 109 is brought into pressure contact against a brake friction disc 114 secured to a bracket 113 to cause an output shaft 106 to come to an abrupt stop. The rotational force of the output shaft 106 is normally transmitted to the machine head through a belt. Speed signals detected by a detector are fed back and the exciting current of a clutch electromagnet 110 is adjusted to cause a half-clutch condition to perform the speed control of the machine. Also, a given-position stop of the machine needle is performed through the excitation of a brake electromagnet 112 with the needle position signal of a detector.
In recent years, higher functions of the industrial machine and development of an automatic machine allows the driving motor to have a stronger tendency of providing not only the conventional driving stop, but also the long hours' operations at an intermediate speed. In the conventional motor shown in FIG. 1, the lining 108 was slid as described hereinabove and was used at the half-clutched condition during an intermediate-speed operation so as to promote the abrasion of the lining, thus resulting in insufficient service life.
A method of applying molybdenum-bisulfide containing grease on the sliding face was put into practical use. Generally, the lining was chiefly composed of cork, including binder such as urethane, and impregnated oil. Considerably longer service life was achieved, but restrictions such as periodical grease application or lining exchange were required. As the reactance of the electromagnet was comparatively large, the stability of the speed was inferior on the side of the clutch and the stop position of the machine needle was likely to be dispersed due to the delay of the exciting current on the side of the brake.
In order to improve the service life, a system with a friction type clutch being replaced by an eddy-current clutch is proposed and put into practical use in some quarters. This system was clearly improved in such clutch service life as described hereinabove to transmit the rotational force of the fly-wheel to the output shaft through non-contact, but no progress was made in the service life of the brake portion, because the brake portion used the friction type brake similar to the conventional one due to insufficient damping force. Also, to prevent the eddy-current producing portion from being thermally deformed, the exciting current had to be restricted, so that sufficient torque could not be provided.
Furthermore, the conventional systems consumed unnecessary power, because the driving motor always had to be rotated in spite of the driving and stopping operations of the machine. Normally, the operation time of the machine was 20 to 30%, the motor was loosely operated in the remaining 70 to 80%, and in a motor of 400W in single phase, the power of about 100W was consumed in vain.
Also, the fly-wheel provided in the driving motor had to be made larger in its outer diameter and heavier in weight because of the requirement of greater rotational energy, thus interfering with possible smaller size and lighter weight of the driving motor.